Patent Document 1 discloses a conventional hologram recording device. The hologram recording device disclosed in this document is configured to apply a recording beam modulated by a spatial light modulation element vertically to a hologram recording medium, and obliquely apply a reference beam so as to superpose the reference beam on the recording beam, to thereby record a hologram by interference between the recording beam and the reference beam. The recording beam and the reference beam are generated by a beam splitter that splits a laser beam emitted by a light source. Between the spatial light modulation element and the hologram recording medium, a telecentric type objective lens is disposed. The telecentric type objective lenses are generally designed to have a low aberration as possible, and the wavefront aberration on a best image plane does not exceed 0.5 λrms, for example.
Patent Document 1: JP-A-07-319372
Such conventional hologram recording device has, however, the following drawback originating from the extremely small aberration of the objective lens.
On the assumption that the best image plane of the recording beam is formed inside the hologram recording medium, and when the illumination area of the recording beam on the best image plane is designated by M and the illumination area of the reference beam on the best image plane by F, in the case where an illumination area ratio defined as F/M becomes smaller than 0.6 or so as shown in FIGS. 5 and 6, errors are frequently incurred at reproduction of the hologram. This can also be clearly understood in view of the S/N ratio tendency obtained through imaging signals.
Such reproduction error is considered to be associated with the image-forming performance of the objective lens. For example, under a condition where the aberration of the objective lens is as close as possible to 0, optically adjusting the amplitude and the light intensity of the recording beam and the reference beam, so as to emphasize the interference fringe of the 0-order image on the best image plane of the recording beam, leads to failure in clearly forming the interference fringe of other images such as a first order or second order image, resulting in failure in forming the overall hologram image in desirable contrast. Naturally, the hologram is not recorded in desirable contrast in regions other than the best image plane. At reproduction of the hologram, the reference beam is also applied to the regions other than the best image plane so as to generate the reproduction beam, and the reproduced image of the hologram is obtained through receiving the reproduction beam with an imaging device. Accordingly, the reproduced image is unclear as a whole since the recorded hologram includes a portion with good contrast and a portion with poor contrast. Thus, excessively reducing the aberration of the objective lens leads to the disadvantage that the reproduction error is increased.